Advancements in MIMO (Multiple Input Multiple Output) technologies using multiple transmitter and receiver antennas are being studied under the LTE standard. For example, in typical downlink MIMO communication, a user equipment (UE) estimates downlink channel states based on downlink reference signals from the base station. The UE reports the estimated downlink channel states as channel state information (CST) feedback information to the base station. The base station then performs link adaptation for downlink data transmission based on the CSI feedback information. Typical link adaptation may include control of the number of spatial multiplexing layers, transmission beam control, and a modulation and coding scheme. A codebook-based precoding scheme and a beam selection-based precoding scheme are described below as examples of closed-loop precoding schemes in the link adaptation using precoding.
FIG. 1 shows a sequence diagram of signal processing of the codebook-based precoding scheme. In the codebook-based precoding, the base station transmits the reference signal for estimating the downlink channel states, (for example, a Channel State Information Reference Signal (CSI-RS)) (step S11). Based on an estimation result of the received reference signal, the UE selects the best weights among predetermined precoding weights candidates (codebook) and provides the best weights as an index (PMI: Precoding Matrix Indicator) to the base station for feedback of the CSI (step S12). In the downlink transmission, the base station transmits a data signal precoded based on the PMI (step S13).
FIG. 2 shows a sequence diagram of an example of signal processing of the beam selection-based precoding scheme. In the beam selection-based scheme, the base station transmits multiple precoded beams (for example, a precoded CSI-RS) (step S21). The UE selects the suitable beam(s) among the precoded beams and provides a result of selection indicating a Beam Index (BI) to the base station for feedback (step S22). The base station transmits the downlink data signal precoded based on the result of selection (step S23).
On the other hand, 3D (three-dimensional) MIMO techniques are being studied under the LTE standard Release 13. The 3D MIMO techniques are capable of transmission beam control in a three-dimensional direction using a 3D MIMO antenna, wherein antenna elements are arranged in vertical and horizontal two-dimensional plane or even three-dimensional space.
The closed-loop precoding schemes as described above requires, as the number of antennas such as the 3D MIMO antenna increases, uplink channels used for the CSI feedback and more resources for the channels used for the CSI feedback. Reciprocity-based link adaptation is being studied for resource reservation for the channels used for the CSI feedback. For example, with reciprocity-based precoding, instead of measuring the downlink channel states as described above, the base station measures uplink channel states and controls downlink beamforming based on the measured uplink channel states. The reciprocity-based precoding is based on an assumption called channel reciprocity that uplink channel states and downlink channel states are approximately equivalent, and the measurement result of the uplink channel states is used instead of the downlink channel states. It is possible to measure the uplink channel states based on an uplink reference signal such as a SRS (Sounding Reference Signal) or a DM-RS (Demodulation Reference Signal) transmitted by the user equipment.
It is possible to consider that CSI is obtained based on both of channel reciprocity and CSI feedback. For example, for beamformed CSI-RS based scheme, CSI-RS can be beamformed based on CSI obtained from channel reciprocity.
To sufficiently assure channel reciprocity as described above, the level of imperfections of transmitter and receiver antenna and RF equipment must be low (accuracy of calibration must be sufficient). If accuracy of calibration is low, accuracy of channel information obtained based on channel reciprocity is lowered, and as a result, a part or all of the channel information may be unusable. Furthermore, if the number of downlink receiver antennas is different from the number of uplink transmitter antennas, the channel state information based on channel reciprocity is obtained only from a partial combination of antennas.
As described above, in system assuring channel reciprocity, uplink channel estimation results can be utilized in link adaptation for downlink transmission. However, link adaptation depending solely on channel reciprocity may not be possible due to RF imperfections and reception quality of reference signals.
Furthermore, link adaptation depending on channel reciprocity only may have limitations in some situations even if RF imperfections do not cause problems and reception quality is sufficiently high. For example, a system performing channel reciprocity-based precoding allows the base station to select a transmission precoding vector or a precoding matrix indicator (PMI) based on the estimated uplink channel state. However, it may be difficult to control the number of multiple layers and the encoded modulation system because the base station cannot estimate channel quality and interference conditions in the user equipment. In addition, as described above, if the number of antennas of uplink and the number of antennas of downlink are asymmetrical, channel state information obtained by channel reciprocity is limited.
Therefore, in a system adopting characteristics of channel reciprocity, accuracy of link adaptation may be lowered due to imperfections and reception quality of reference signals of an RF circuit and an antenna system. In addition, in a case where channel reciprocity is used, it may be difficult to perform adaptive control of the number of multiple layers and the encoded modulation system because the base station cannot estimate received power and interference conditions in the user equipment.